ALBERT

Description: Fires affect the composition of the atmosphere and Earth's radiation balance by emitting a suite of reactive gases and particles. An interactive fire module in an Earth system model (ESM) allows us to study the natural and anthropogenic drivers, feedbacks, and interactions of open fires. To do so, we have developed pyrE, the NASA GISS (Goddard Institute for Space Studies) interactive fire emissions module. The pyrE module is driven by environmental variables like flammability and cloud-to-ground lightning, calculated by the GISS ModelE ESM, and parameterized by anthropogenic impacts based on population density data. Fire emissions are generated from the flaming phase in pyrE (active fires). Using pyrE, we examine fire occurrence, regional fire suppression, burned area, fire emissions, and how it all affects atmospheric composition. To do so, we evaluate pyrE by comparing it to satellite-based datasets of fire count, burned area, fire emissions, and aerosol optical depth (AOD). We demonstrate pyrE's ability to simulate the daily and seasonal cycles of open fires and resulting emissions. Our results indicate that interactive fire emissions are biased low by 32 %–42 %, depending on emitted species, compared to the GFED4s (Global Fire Emissions Database) inventory. The bias in emissions drives underestimation in column densities, which is diluted by natural and anthropogenic emissions sources and production and loss mechanisms. Regionally, the resulting AOD of a simulation with interactive fire emissions is underestimated mostly over Indonesia compared to a simulation with GFED4s emissions and to MODIS AOD. In other parts of the world pyrE's performance in terms of AOD is marginal to a simulation with prescribed fire emissions.

Description: A new capture vaporizer (CV) has been developed and used recently in the Aerodyne aerosol mass spectrometer (AMS) and aerosol chemical speciation monitor (ACSM) instead of the standard vaporizer (SV) to reduce the particle bounce. It is important to characterize the CV performance in different environments. In this study, we characterized specific organic aerosols (OAs) from vehicle, cooking, biomass burning, and coal burning emissions by a time-of-flight ACSM (TOF-ACSM) with the CV. Their corresponding marker ions that have been defined in the previous SV-based analysis are still valid in the CV mass spectra. Spectra of OAs from cooking and vehicle exhaust show similarities in distinct alkyl fragments but different ratios of m∕z 55 and 57. Ions related to polycyclic aromatic hydrocarbons are present in the OA spectra obtained from burning lignite and bituminous coal but not in the spectra obtained from burning anthracite. Although the relative intensities of m∕z 60 and 73 are much lower in the CV spectra than in the SV spectra for biomass burning OA, they are still relatively greater compared with the spectra for other sources. Our data suggest an atmospheric background of f60 of below 0.03 % for CV. Moreover, we deployed the CV TOF-ACSM along with a SV AMS in urban Beijing during the winter of 2017 to characterize ambient OA with strong anthropogenic influences. The CV TOF-ACSM shows a collection efficiency (CE) of about unity. The CV and SV data show consistent mass concentrations of sulfate, nitrate, ammonium, and OA. Six OA factors are identified by the positive matrix factorization (PMF) analysis for both the CV and the SV data. The SV and CV PMF factors show good correlations in mass concentrations. The SV and CV factors related to coal combustion and cooking differ significantly in loadings, explained by the PMF uncertainty and the lack of understanding of the relative ionization efficiency and CE for primary OA. The CV factors related to secondary sources show greater loadings than the SV factors, which may be associated with the changes in signal-to-noise ratios of various ions in the PMF analysis. Our results support improved mass quantification and useful source identification by the CV for ambient particles in the polluted urban environment. The difference in factor loadings between SV and CV should be considered when interpreting or comparing the PMF results among studies.

Description: While hydroxyl radical (·OH) in the snowpack is likely a dominant oxidant for organic species and bromide, little is known about the kinetics or steady-state concentrations of ·OH on/in snow and ice. Here we measure the formation rate, lifetime, and concentration of ·OH for illuminated polar snow samples studied in the laboratory and in the field. Laboratory studies show that ·OH kinetics and steady-state concentrations are essentially the same for a given sample studied as ice and liquid, in contrast to other oxidants, which show a concentration enhancement in ice relative to solution. The average production rate of ·OH in samples studied at Summit, Greenland is 5 times lower than the average measured in the laboratory, while the average ·OH lifetime determined in the field is 5 times higher than in the laboratory. These differences indicate the polar snows studied in the laboratory are affected by contamination, despite efforts to prevent this. Steady-state concentrations of ·OH in snow studied in the field at Summit, Greenland range from (0.8 to 3) × 10−15 M, comparable to values reported for mid-latitude cloud and fog drops, rain, and deliquesced marine particles, even though impurity concentrations in the snow samples are much lower. Partitioning of firn-air ·OH to the snow grains will approximately double the steady-state concentration of snow-grain hydroxyl radical, leading to an average [·OH] in near-surface, summer Summit snow of approximately 4 × 10−15 M. At this concentration, the ·OH-mediated lifetimes of organics and bromide in Summit snow grains are approximately 3 days and 7 hours, respectively, suggesting that hydroxyl radical is a major oxidant for both species.

Description: The spatial distribution of aerosols and their chemical composition dictates whether aerosols have a cooling or a warming effect on the climate system. Hence, properly modeling the 3-dimensional distribution of aerosols is a crucial step for coherent climate simulations. Since surface measurement networks only give 2-D data, and most satellites supply integrated column information, it is thus important to integrate aircraft measurements in climate model evaluations. In this study, the vertical distribution of secondary inorganic aerosol (i.e. sulfate, ammonium and nitrate) is evaluated against a collection of 14 AMS flight campaigns and surface measurements from 2000–2010 in the USA and Europe. GISS ModelE2 is used with multiple aerosol microphysics (MATRIX, OMA) and thermodynamic (ISORROPIA II, EQSAM) configurations. Our results show that the MATRIX microphysical scheme improves the model performance for sulfate, but that there is a systematic underestimation of ammonium and nitrate over the USA and Europe in all model configurations. In terms of gaseous precursors, nitric acid concentrations are largely underestimated at the surface while overestimated in the higher levels of the model, influenced by strong stratosphere-troposphere exchange. Heterogeneous reactions on dust surfaces is an important sink for nitric acid, even high in the troposphere. At high altitudes, nitrate formation is calculated to be ammonia limited. The underestimation of ammonium and nitrate in polluted regions is most likely caused by a too simplified treatment of the NH3/NH4+ partitioning which affects the HNO3/NO3− partitioning.

Description: The operational Hurricane Weather Research and Forecast (HWRF) model has been used to investigate the role of lightning diagnostics in the life cycle of tropical cyclones. A lightning parameterization, the Lightning Potential Index (LPI), was implemented into HWRF with the motivation that an improvement in the forecast of lightning will lead to reductions in the HWRF model intensity forecast errors and bias. Three questions are addressed: (i) Can the HWRF model predict lightning temporal distributions with an acceptable degree of accuracy? (ii) How well does the HWRF model with lightning parameterization forecast lightning spatial distributions before, during, and after tropical cyclone intensification? (iii) What is the functional relationship between tropical cyclone wind speed and lightning frequency in the HWRF model forecast? A five-day simulation of Idealized tropical cyclones with and without eyewall replacement cycle, has been conducted, followed by two real cases e.g. hurricanes Earl and Igor to evaluate the evolution of the spatial distribution of lightning location. Results from this investigation led to the following observations: (1) the potential for lightning occurrence increases to its maximum peak prior to the maximum predicted wind intensity and (2) the numerical simulations predict a negative correlation between lightning occurrence and maximum winds during the storm’s peak intensity.

Description: While the hydroxyl radical (•OH) in the snowpack is likely a dominant oxidant for organic species and bromide, little is known about the kinetics or steady-state concentrations of •OH on/in snow and ice. Here we measure the formation rate, lifetime, and concentration of •OH for illuminated polar snow samples studied in the laboratory and in the field. Laboratory studies show that •OH kinetics and steady-state concentrations are essentially the same for a given sample studied as ice and liquid; this is in contrast to other photooxidants, which show a concentration enhancement in ice relative to solution as a result of kinetic differences in the two phases. The average production rate of •OH in samples studied at Summit, Greenland, is 5 times lower than the average measured in the laboratory, while the average •OH lifetime determined in the field is 5 times higher than in the laboratory. These differences indicate that the polar snows we studied in the laboratory are affected by contamination, despite significant efforts to prevent this; our results suggest similar contamination may be a widespread problem in laboratory studies of ice chemistry. Steady-state concentrations of •OH in clean snow studied in the field at Summit, Greenland, range from (0.8 to 3) × 10−15 M, comparable to values reported for midlatitude cloud and fog drops, rain, and deliquesced marine particles, even though impurity concentrations in the snow samples are much lower. Partitioning of firn air •OH to the snow grains will approximately double the steady-state concentration of snow-grain hydroxyl radical, leading to an average [•OH] in near-surface, summer Summit snow of approximately 4 × 10−15 M. At this concentration, the •OH-mediated lifetimes of organics and bromide in Summit snow grains are approximately 3 days and 7 h, respectively, suggesting that hydroxyl radical is a major oxidant for both species.

Description: The spatial distribution of aerosols and their chemical composition dictates whether aerosols have a cooling or a warming effect on the climate system. Hence, properly modeling the three-dimensional distribution of aerosols is a crucial step for coherent climate simulations. Since surface measurement networks only give 2-D data, and most satellites supply integrated column information, it is thus important to integrate aircraft measurements in climate model evaluations. In this study, the vertical distribution of secondary inorganic aerosol (i.e., sulfate, ammonium, and nitrate) is evaluated against a collection of 14 AMS flight campaigns and surface measurements from 2000 to 2010 in the USA and Europe. GISS ModelE2 is used with multiple aerosol microphysics (MATRIX, OMA) and thermodynamic (ISORROPIA II, EQSAM) configurations. Our results show that the MATRIX microphysical scheme improves the model performance for sulfate, but that there is a systematic underestimation of ammonium and nitrate over the USA and Europe in all model configurations. In terms of gaseous precursors, nitric acid concentrations are largely underestimated at the surface while overestimated in the higher levels of the model. Heterogeneous reactions on dust surfaces are an important sink for nitric acid, even high in the troposphere. At high altitudes, nitrate formation is calculated to be ammonia limited. The underestimation of ammonium and nitrate in polluted regions is most likely caused by a too simplified treatment of the NH3 ∕ NH4+ partitioning which affects the HNO3 ∕ NO3− partitioning.

Description: As critical precursors of ozone (O3) and secondary organic aerosols, volatile organic compounds (VOCs) play a vital role in air quality, human health, and climate change. In this study, a campaign of comprehensive field observations and VOC grid sampling was conducted in Xi'an, China, from 20 June to 20 July 2019 to identify the spatiotemporal concentration levels, sources, and secondary transformation potential of VOCs. During the observation period, the average VOC concentrations at the Chanba (CB), Di Huan Suo (DHS), Qinling (QL), and gridded sampling sites were 27.8 ± 8.9, 33.8 ± 10.5, 15.5 ± 5.8, and 29.1 ± 8.4 ppb, respectively. Vehicle exhaust was the primary source of VOC emissions in Xi'an, and the contributions of vehicle exhaust to VOCs at the CB, DHS, and QL sites were 41.3 %, 30.6 %, and 23.6 %–41.4 %, respectively. While industrial emissions were the second-largest source of VOCs in urban areas, contributions from aging sources were high in rural areas. High potential source contribution function values primarily appeared in eastern and southern Xi'an near the sampling site, which indicates that Xi'an exhibits a strong local VOC source. Moreover, alkenes, aromatics, and oxygenated VOCs played a dominant role in secondary transformation, which is a major concern in reducing O3 pollution in Xi'an.

Description: Oxidation of aromatic volatile organic compounds (VOCs) leads to the formation of tropospheric ozone and secondary organic aerosol, for which gaseous oxygenated products are important intermediates. We show, herein, the experimental results of highly oxygenated organic molecules (HOMs) produced by the oxidation of benzene and toluene in a wide range of OH exposure and NOx conditions. The results suggest that multigeneration OH oxidation plays an important role in the product distribution, which likely proceeds more preferably via H subtraction than OH addition for early generation products from light aromatics. More oxygenated products present in our study than in previous flow tube studies, highlighting the impact of experimental conditions on product distributions. The formation of dimeric products, however, was suppressed and might be unfavorable under conditions of high OH exposure and low NOx in toluene oxidation. Under high-NOx conditions, nitrogen-containing multifunctional products are formed, while the formation of other HOMs is suppressed. Products containing two nitrogen atoms become more important as the NOx level increases, and the concentrations of these compounds depend significantly on NO2. The highly oxygenated nitrogen-containing products might be peroxyacyl nitrates, implying a prolonged effective lifetime of RO2 that facilitates regional pollution. Our results call for further investigation on the roles of high-NO2 conditions in the oxidation of aromatic VOCs.